The present invention relates, in general, to three-terminal, semiconductor solid state devices such as bipolar transistors, permeable base transistors, and planar doped barrier transistors, and more particularly to such a device wherein the emitter-base barrier is so constructed as to permit near ballistic electron motion, and to thereby provide a device capable of very high speed operation.
In a semicondutor device having a "vertical" structure, e.g., where current flows from the emitter through the base to the collector, and where the current flow is modulated by the base current or the base voltage, limitations are encountered in the higher frequency ranges because of the diffusion mechanism by which the charge carriers move through the device. It is known that diffusion limitations can be reduced by decreasing the thickness of the base region, but such a decrease also undersirably increases base resistance. In addition, such a decrease in thickness does not entirely eliminate diffusion mechanism limitations because in the usual bipolar heterojunction device, electrons are injected into the base region with a low (about 26 meV) thermal energy; thus, high frequency operation of such devices, particularly in the microwave frequency range, has not been entirely successful.
U.S. Pat. No. 3,209,215 to Esaki is an early patent disclosing heterojunction semiconductor devices, wherein a narrow base region is provided, but where the emitter, base and collector are of the same conductivity type. This patent discloses a structure wherein tunneling effects are produced, however, and no consideration of the utilization of a heterojunction triode with a base of opposite conductivity is disclosed.
U.S. Pat. No. 4,286,275 to Heiblum also relates to a three-terminal semiconductor device and is directed to a construction for obtaining increased switching speed. The patent recognizes the limitations of diffusion motion of the charge carriers, and solves the problem by selecting the barrier height and width for the emitter (or carrier injection region) so that the dominant current injection mechanism is that of tunneling.
U.S. Pat. No. 4,173,763 to change et al also relates to a heterojunction transistor in which a base thickness of 100.ANG. or less is used to produce tunneling from the emitter to the collector, in order to provide a high speed device.
Although these prior art patents illustrate that the limitations of the diffusion mechanism are known, and that attempts have been made to overcome them through various structures in order to produce transistors which operate at higher frequencies, the prior art does not suggest that ballistic or near ballistic transport of carriers could be used to improve the speed of operation.
Although ballistic motion in a thin base of a Ga As bipolar transistor has been suggested by Rohr et al "Solid State Electronics," 1974, Vol. 17, pp. 729-734, they did not take into account the fact that high energy motion can lead to scattering effects, wherein electrons are scattered to upper valleys, did not recognize that potential barriers could be used to launch electrons, and thus overlooked the fact that such motion could be utilized to solve the problem of diffusion limitations to obtain high speed operation.